Multilayered structures such as fiber Bragg gratings (FBG) are used in a diverse range of applications in telecommunication systems, fiber optic sensors and fiber lasers. It is well known that their performance in these applications is strongly affected by their reflectivity and dispersion characteristics. However, during their inscription, instabilities in the UV fringe pattern and imperfections in the medium containing the FBG can induce errors in the periodic structure written into the medium. The existence of such errors in the phase and amplitude of the refractive index modulation forming the grating can cause significant deviation of these characteristics from the target or required characteristics. See R. Feced, and M. N. Zervas, “Effects of random phase and amplitude errors in optical fiber Bragg gratings,” J. of Lightw. Technol., Vol. 18, No. 1, pp. 90-101 (2000). These errors are usually imparted during the writing process. Consequently, methods to characterize the refractive index profile of the Bragg grating post-inscription are extremely desirable to help understand the source of these errors, assist in optimization of the FBG inscription process, and provide quality control.
Techniques to characterize the spatial profile of FBGs post-inscription can be divided into the following groups: (a) swept frequency or low coherence interferometry and layer peeling, (b) side-scattering and (c) local perturbation (i.e. heat or stress) methods. None of these techniques are capable of characterizing all grating types. Thus, each has to be judged on its own merits when attempting to characterize a given FBG.
The spatial profile of the grating is described by its complex coupling coefficient q(z). See T. Erdogan, “Fiber grating spectra” J. Lightw. Technol., Vol. 15, No. 8, pp. 1277-1294 (1997). The coupling coefficient q(z) is difficult to measure or calculate directly, so methods have been developed to calculate it indirectly. For example, swept frequency methods such as optical frequency domain reflectometry (OFDR) and low coherence methods such as optical low-coherence reflectometry measure the impulse response of the grating, which is used with an inverse scattering technique such as layer peeling to reconstruct the complex coupling coefficient of the grating. See P. Giaccari, H. G. Limberger, and R. P. Salathe, “Local coupling-coefficient characterization in fiber Bragg gratings,” Optics Letters, Vol. 28, No. 8, pp. 598-600 (2003); and X. Chapeleau, D. Leduc, C. Lupi, F. Lopez-Gejo, M. Douay, R. Le Ny, and C. Boisrobert, “Local characterization of fiber-Bragg gratings through combined use of low-coherence interferometry and a layer-peeling algorithm,” Applied Optics, Vol. 45, No. 4, pp. 728-735 (2006).
These methods are capable of achieving very high spatial resolution measurements (<100 μm). However, these techniques will always fail as the grating strength increases (qL>˜4) such that the on-resonance penetration depth of the incident optical field is not sufficient to provide an adequate signal-to-noise ratio for the recursive reconstruction algorithm to be stable. By applying a well controlled thermally induced chirp to the grating, the use of these techniques has been extended to stronger gratings with qL=8.25. See O. H. Waagaard, “Spatial characterization of strong fiber Bragg gratings using thermal chirp and optical-frequency-domain reflectometry”, J. Lightw. Technol., Vol. 23, No. 2, pp. 909-914 (2005). However, an upper limit still remains due to practical limitation of the thermal gradient that can be applied to the grating (˜150K).
The side scattering technique measures the diffracted power from the grating when illuminated from the side and is well suited to characterizing highly chirped gratings such as dispersion compensators. This technique is capable of measuring the amplitude of the refractive index modulation, P. A. Krug, R. Stolte, and R. Ulrich, “Measurement of index modulation along an optical fiber Bragg grating,” Optics Letters, Vol. 20, No. 17, pp. 1767-1769 (1995), and its phase, F. El-Diasty, A. Heaney, and T. Erdogan, “Analysis of fiber Bragg gratings by a side-diffraction interference technique,” Applied Optics, Vol. 40, No. 6, pp. 890-896 (2001); I. Petermann, S. Helmfrid, and A. T. Friberg, “Limitations of the interferometric side diffraction technique for fibre Bragg grating characterization,” Optics Communications 201 (2002) 301-308, but requires careful alignment and a high quality fiber surface to obtain reliable phase data. To date, reproducibility errors of 2-5% for the amplitude and 10°-20° for the phase have been reported. I. Petermann, S. Helmfrid, and P. Y. Fonjallaz, “Fibre Bragg grating characterization with ultraviolet-based interferometric side diffraction,” J. Opt. A: Pure Appl. Opt. 5 (2003) 437-441. These errors can be reduced by spatially averaging the measurement at the expense of a reduced spatial resolution (few mm).
A second type of side-scattering method measures the Rayleigh scattered power radiated from the side of the grating when interrogated through the core. J. Canning, M. Janos, D. Y. Stepanov, and M. G. Sceats, “Direct measurement of grating chirp using resonant side scatter spectra,” Electronics Letters, Vol. 32, No. 17, pp. 1608-1610 (1996); J. Canning, D. C. Psaila, Z. Brodzeli, A. Higley, and M. Janos, “Characterization of apodized fiber Bragg gratings for rejection filter applications”, Applied Optics, Vol. 36, No. 36, pp. 9378-9382 (1997). However, a detailed analysis of this second side-scattering method has not been presented to date.
Perturbation methods involve locally modifying the refractive index of the fiber in a controlled way. This causes a change in the FBG's spectral properties, which are measured in some way and related to the grating properties at the location of the perturbation. The perturbation can be applied by a local pressure, see C. J. S. de Matos, P. Torres, L. C. G. Valente, W. Margulis, and R. Stubbe, “Fiber Bragg grating (FBG) characterization and shaping by local pressure,” J. of Lightw. Technol., Vol. 19, No. 8, pp. 1206-1211 (2001); by scanning a heated wire, see N. Roussel, S. Magne, C. Martinez, and P. Ferdinand, “Measurement of index modulation along fiber Bragg gratings by side scattering and local heating techniques,” Optical Fiber Technology 5, 119-132 (1999); by scanning the fiber with a He—Ne laser, see E. Brinkmeyer, G. Stolze, D. Johlen, “Optical space domain reflectometry (OSDR) for determination of strength and chirp distribution along optical fiber gratings” Proc. OSA Conf Bragg Gratings 1996, (Photosensitivity and Poling in Glass Waveguides (BGPP) 1997), paper BsuC2-1, p. 33; or by scanning the fiber with a CO2 laser, see I. G. Korolev, S. A. Vasil'ev, O. I. Medvedkov, and E. M. Dianov, “Study of local properties of fibre Bragg gratings by the method of optical space-domain reflectometry,” Quantum Electronics 33(8), 704-710 (2003). Physical contact with the fiber is likely to affect its mechanical integrity, thus non-contact methods such as those using a carbon dioxide (CO2) laser are preferable.
Of these heat scan techniques, the method of optical space domain reflectometry (OSDR) has many favorable attributes. It is capable of extracting the complex coupling coefficient of the grating from a measurement at a single wavelength; it requires only to heat the grating by a few degrees Kelvin, which can be implemented with a focused beam from a CO2 laser, see Korolev, et al., supra, and does not require any complex alignment of the fiber other than to align the heating beam. Previous implementations of this technique, however, have yielded poor accuracy of the measurement. See Brinkmeyer, et al., supra and Korolev, et al., supra.
In addition, although characterization of the refractive index profile of the Bragg grating post-inscription can aid in understanding the source of inscription errors, assist in optimization of the FBG inscription process, and provide quality control, characterization of the multilayered structure during the inscription process (i.e. real time, in-situ monitoring) is even more desirable, as it can enable implementation of a closed loop inscription system which can prevent errors from building up, yielding an error-free structure from the outset. Thus, in order to achieve error free periodic structures written into the media, it is desirable to perform an in-situ measurement of the multilayered structure during the inscription procedure.
The basic concept of using interferometry to track the complex reflectivity of a multilayered structure and reconstruct the local coupling coefficient has been suggested in, for example, U.S. Pat. No. 7,194,163 to Stepanov, “Multilayered structure characterization”; and D. Stepanov, G. Edvell, and M. Sceats, “Monitoring of the fiber Bragg grating fabrication process,” in Optical Fiber Communication Conference, Technical Digest (CD) (Optical Society of America, 2004), paper ThC1. However, the methods published therein fail to show how the interferometric measurement can reject spurious signals due to parasitic cavities present in all interferometric configurations or how they can achieve sufficient sensitivity to resolve to the level of Rayleigh scattering. Furthermore, the published method for reconstructing the coupling coefficient from the complex reflectivity from a linearly proportional relationship between these two quantities has been shown to be invalid in almost all practical situations. Other interferometric methods are capable of measuring the complex reflectivity to the limit of Rayleigh scattering while rejecting signals from spurious reflections such as Optical Frequency Domain Reflectometry (OFDR). See, e.g., S. Kieckbusch, Ch. Knathe, and E. Brinkmeyer, “Fast and Accurate Characterization of Fiber Bragg Gratings with High Spatial and Spectral Resolution”, OFC 2003, vol. 1, p. 379 (2003). However, these methods gather far more data than is necessary to implement the above technique and therefore are considerably slower. Also, since OFDR requires a scanned laser source, it is not a continuous measurement, and thus is not capable of tracking the spectral phase of the multilayered structure.